A certain element has a half life of 4.5 billion years. a. You find a rock containing a mixture of the element and lead. You determine that 60​% of the original element​ remains; the other 40​% decayed into lead. How old is the​ rock? b. Analysis of another rock shows that it contains 5​% of its original​ element; the other 95​% decayed into lead. How old is the​ rock?